1. Field of the Invention
The present invention relates to a propeller unit for a marine vessel propulsion device arranged to be attached to a propeller shaft driven by an engine or other motor and relates to a marine vessel propulsion device that includes the propeller unit.
2. Description of Related Art
A propeller unit of a marine vessel propulsion device includes, for example, an inner cylinder fixed to a propeller shaft, an outer cylinder surrounding the inner cylinder, and a cylindrical damper made of rubber and disposed between the inner cylinder and the outer cylinder. The damper is fixed to the inner cylinder, and the damper that is made integral with the inner cylinder is press fitted into the outer cylinder. A driving force transmitted from the engine to the propeller shaft is transmitted via the inner cylinder and the damper to the outer cylinder and rotates blades fixed to the outer cylinder. The blades thereby push the water and a propulsive force that propels the marine vessel is generated.
An outboard motor, which is one example of a marine vessel propulsion device, may be provided with a shift mechanism for switching a direction of the propulsive force between a forward drive direction and a reverse drive direction. A typical shift mechanism includes a dog clutch, a forward drive gear, and a reverse drive gear. The dog clutch is spline-connected to the propeller shaft and is arranged to be selectively coupled to the forward drive gear and the reverse drive gear. A rotation of a driveshaft that transmits the driving force from the engine is constantly transmitted to the forward drive gear and the reverse drive gear. The forward drive gear and the reverse drive gear are arranged to receive the driving force from the driveshaft and rotate in mutually opposite directions. When the dog clutch is coupled to the forward drive gear, the propeller unit rotates in a direction of generating a propulsive force that drives the marine vessel forward. When the dog clutch is coupled to the reverse drive gear, the propeller unit rotates in a direction of generating a propulsive force that drives the marine vessel in reverse. When the dog clutch is not coupled to either of the forward drive gear and the reverse drive gear, the driving force of the engine is not transmitted to the propeller unit.
An operation by which the dog clutch becomes coupled the forward drive gear or the reverse drive gear is called “shift-in,” and an operation in which the dog clutch is released from being coupled to the forward drive gear or the reverse drive gear is called “shift-out.” Hereinafter, the shift-in and shift-out operations shall be referred to collectively as “shift switching.” During shift switching, a shock (shift shock) occurs in the dog clutch and the propeller shaft. A main cause of the shift shock is a large inertial mass of the propeller unit. For example, when shift-in occurs while the rotation of the propeller unit is stopped, the dog clutch that is in a rotation-stopped state engages with the forward drive gear or the reverse drive gear that is being rotated by the engine. The dog clutch is spline-connected to the propeller shaft, and the propeller unit with the large inertial mass is coupled to the propeller shaft. A strong impact thus occurs when the forward drive gear or the reverse drive gear becomes coupled to the dog clutch. During shift-out, although a shift shock occurs because the large inertial mass is cut off from the engine, the shift shock is not as strong as that during shift-in.
The shift shock can be reduced by making the damper rubber of the propeller unit soft. For example, the shift shock is alleviated by a soft damper being strongly twisted between the outer cylinder and inner cylinder during shift-in.
However, it is insufficient for the damper to just be able to alleviate the shift shock and it must also be able to transmit the driving force of the propeller shaft to the blades. More specifically, the damper must transmit the driving force of the propeller shaft from the inner cylinder to the outer cylinder without undergoing rupture, slipping, etc., between the inner cylinder and the outer cylinder. For this purpose, it is desirable for the damper to be as hard as possible.
Thus, generally, a material of the damper is determined in consideration of balancing both a shift shock reducing function and a driving force transmitting function. It was thus difficult to improve both functions at the same time.
A propeller unit, with which a damper provides a shift shock reducing function and a driving force of a propeller shaft is transmitted to an outer cylinder mainly by a hard material except the damper, is described in US 2007/053777A1. This propeller unit includes an inner cylinder fixed to the propeller shaft, the outer cylinder to which blades are fixed, an intermediate cylinder arranged between the inner cylinder and the outer cylinder, and a cylindrical damper disposed between the intermediate cylinder and the inner cylinder. The shift shock is alleviated by elastic deformation of the damper between the inner cylinder and the intermediate cylinder. When the driving force of the propeller shaft is small, the driving force of the propeller shaft is transmitted in the order of the inner cylinder, the damper, the intermediate cylinder, the outer cylinder, and the blades. When the driving force of the propeller shaft becomes not less than a predetermined value, an elastic deformation amount of the damper increases and the intermediate cylinder rotates by a predetermined angle with respect to the inner cylinder. In this process, a first engaging member formed at a rear end of the intermediate cylinder engages with a second engaging member formed at a rear end of the inner cylinder and the intermediate cylinder and the inner cylinder are thereby coupled rigidly. The driving force of the propeller shaft is thus transmitted in the order of the second engaging member of the inner cylinder, the first engaging member of the intermediate cylinder, the outer cylinder, and the blades, and a large driving force is not transmitted to the damper. The driving force can thus be transmitted from the propeller shaft to the blades without dependence on the rigidity of the damper.
With the propeller unit for marine vessel propulsion device described in US 2007/053777 A1, a torque that is transmitted between the intermediate cylinder and the outer cylinder is restricted to no more than a predetermined torque by a tolerance ring arranged between the intermediate cylinder and the outer cylinder. Thus, when the blades receive a strong impact temporarily, the outer cylinder rotates idly with respect to the intermediate cylinder. Application of an impact load to the power transmission system between the propeller shaft and the engine is thereby prevented.